In humans, one of the side effects of chronic chemical antidepressant treatment is a change in body mass. The mechanism of these induced changes is not clear but may include alterations in metabolic rate, as well as effects on specific body components such as body lipid, protein or water. We have previously noted chemical antidepressant induced alterations in the body mass of Syrian hamsters being treated chronically with the monoamine oxidase inhibitor (MAOI) clorgyline. This project was undertaken to more formally explore the behavioral and physiological mechanisms responsible for these alterations in Syrian hamsters. We have observed that initial clorgyline treatment decreases food intake. During chronic drug administration, clorgyline administration reduces the rate of weight gain although the level of food intake is not different from control. Thus, drug treated hamsters consume more food per unit of body mass than do control animals, i.e., they exhibit a negative energy balance relative to control animals. Analysis of the carcass composition of clorgyline treated hamsters indicates that drug induced changes are due to a decrease in body lipid content, rather than protein or water. SINCE seasonal changes in body mass are due to fluctuations in body lipid content, our results indicate that clorgyline may be affecting the same photoperiodic process which is responsible for photoperiodic-induced changes in body mass in the Syrian hamster.